Heterogeneous sintered alloys



, 3,142,893 HETEROGENEOUS SINTERED ALLGYS Ernest J. Bradbury, Solihull, Dennis W. Wakeman, Moseley, Birmingham, and Victor Allen Tracey, Solihull, England, assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed June 19, 1962, Ser. No. 203,465 Claims priority, application Great Britain June 20, 1961 Claims. (Cl. 29-182) The present invention relates to sintered alloys and, more particularly, to a process for producing sintered alloys by powder metallurgical techniques.

It is Well known that present-day technological advancements are controlled to a great extent by the materials of construction. For example, present-day aerospace and power plant developments call for materials that have good room temperature and elevated temperature properties and/or characteristics, e.g., good strength at room temperatures and resistance to creep at elevated temperatures. In alloys, the strength and particularly the resistance to creep are largely dependent on the presence of barriers to dislocation movements across the slip planes. Such slip-inhibiting barriers are commonly present as precipitated and/or non metallic dispersed phases. In addition, the barriers are also said to exist in cast alloys as the result of lack of homogeneity in the alloys as cast.

The presence of barriers, e.g., as a precipitate, a nonmetallic dispersion and/or an inhomogeneity, to dislocation movements has created problems that have troubled the art and have somewhat impeded the art in its quest for still better alloys. For instance, precipitation-hardened alloys made by conventional melting and casting procedures oftentimes exhibit weaknesses which are attributable to overaging, i.e., the precipitate tends to agglomerate when the alloy is exposed to temperatures in the aging temperature range for prolonged periods. The correction for such an occurrence involves subjecting the overaged alloy to the entire aging treatment again. This is, of course, uneconomical.

In view of the foregoing, the art has, in many situations, turned to alloys hardened by the incorporation of nonmetallic refractory particles, e.g., alumina, thoria, etc., into the alloy to produce a dispersion-hardened alloy.

The problems appurtenant to barriers in the form of inhomogeneities in cast alloys is that, in the course of working and fabrication, the alloy increases in homogeneity and its properties of strength suffer correspondingly.

Although many attempts were made to overcome the foregoing difficulties and other disadvantages, none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that alloys containing barriers to dislocation movements may be produced by specially controlled powder metallurgical operations.

It is an object of the present invention to provide a process for producing an alloy which has more barriers to dislocation movement in its wrought form than would normally be present having regard to the overall chemical composition and metallurgical structure.

Another object of the present invention is to provide novel alloys having good resistance to creep at elevated temperatures.

Still another object of the present invention is the provision of sintered alloys having barriers to dislocation movements included therein.

It is also an object of this invention to provide a unique powder metallurgical process for making heterogeneous alloys.

State I ten I Other objects and advantages will become apparent from the following description:

Generally speaking, the present invention contemplates a unique powder metallurgical process for producing novel sintered heterogeneous alloys. According to this invention, about 5% to about by weight, of at least one alloy powder is mixed with about 95% to about 5%, by weight, of another alloy powder, sintered at a temperature at least about C. below the melting point of the alloy powder having the lowest melting point for about 2 hours to about 16 hours so that substantial diffusion of the alloy powders does not take place, and then hot worked or otherwise consolidated to form a heterogeneous alloy.

One of the novel features of the invention is the use of a mixture of powders of tWo or more, advantageously two, preformed alloys of different composition in particle sizes such that even after such diffusion as inevitably occurs in the course of the sintering and hot working steps the particles essentially retain their identity and remain different in composition. The particle size of each of the alloy powders is in general at least about 70 microns to about 160 microns, e.g., 100 microns. An admixture of smaller particles up to about 20% by weight of the whole may be made.

In the art of powder metallurgy it is known to alloy powders together and, in order to render the composition of the alloy as uniform as possible, it is usual to start with powders that are as fine as possible. In the commercial grades, the normal maximum size is about 200 microns so that the particles require careful sieving to produce particles of sufficient fineness. In the present invention, however, we are concerned with the production of an alloy that is not uniform even after hot working and heat treatment and this involves the use of relatively large particles, i.e., of the order of 100 microns.

The powders that are mixed together must broadly be compatible with one another. Thus, the matrices of each of the alloys that are to be mixed together are of the same crystal structure, e.g., each has a matrix of face-centered cubic crystal structure. In addition, the melting points are much the same and should not differ by more than about C., i.e., the difference, if any, in melting points is less than about 150 C. Their strength properties, e.g., resistance to creep and resistance to thermal shock, are also much the same.

Broadly, in the final alloy the average lattice parameter of the one component alloy should differ from the other component alloy by at least 0.01% after fabrication and heat treatment. In view of the inevitable diffusion at the surfaces of contact of adjacent particles of different composition with resultant reduction in the initial difference between the lattice parameters at least in the outer zones of these particles, it is necessary to choose alloys having an average difference somewhat greater than about 0.01% and, advantageously, somewhat greater than about 0.1%. This initial difference is, however, less than about 5%.

In carrying the invention into practice, it is advantageous to miX about 20% to about 80%, by weight, of one alloy powder with about 80% to about 20%, by weight, of another alloy powder, each of the alloy powders having the same crystal structure, particle sizes of between about 70 microns and about microns, melting points within about 15 0 C. of each other, and lattice parameters within about 0.1% to about 5% of each other. The mixture is then compacted and sintered and consolidated to form a heterogeneous alloy having good high temperature properties and/ or characteristics together with good room temperature strength properties and/ or characteristics.

The invention is particularly useful in the production of creep-resisting alloys having a face centered cubic structure and a base of one or more of the elements nickel or cobalt or iron. For instance, one well-known creep-resisting alloy contains 20% chromium, 17% cobalt, 2.5% titanium, 1.3% aluminium, 0.08% carbon and small amounts of boron and zirconium, the balance being nickel. Another such alloy contains 15% chromium, 20% cobalt, 1.2% titanium, 4.6% aluminium, molybdenum, 0.15% carbon and the same small amounts of boron and zirconium, with the balance nickel. A typical creep-resisting property of the first of these alloys is rupture in 100 hours at 11 long tons per square inch (t.s.i.) and 870 C. and of the second of the alloys is rupture in 100 hours at 12.8 t.s.i. and 870 C. The lattice parameters of these two alloys differ by about 0.3%. These two alloys may be used to form the initial powder mixture in the invention. Again, the first of these alloys may be mixed with one containing 25% chromium, nickel, 7.5% tungsten and 0.5% carbon, with the balance substantially all cobalt.

The powders may be produced in any convenient way, e.g., by spraying the molten alloy from a nozzle into an inert gas stream. They should, of course, be formed into as intimate a mixture as possible before being sintered.

The heterogeneous alloys of the present invention contain, by weight, about 5% to about 95% of at least one homogeneous alloy substantially uniformly dispersed in about 95% to about 5% of another homogeneous alloy and metallurgically bonded thereto. The lattice parameters of each of the homogeneous alloys are within about 0.01% to about 5% of each other, and, advantageously, about 0.1% to about 5% of each other. The crystal structures of the homogeneous alloys are the same. In addition, the melting points of each of the homogeneous alloys are within about 150 C. of each other.

Advantageously,-the final heterogeneous alloys consist of, by weight, about 20% to about 80% by weight of one homogeneous alloy with the balance another homogeneous alloy having a melting point, a lattice parameter and a crystal structure as hereinbefore set forth.

For the purpose of giving those skilled in the art a better understanding of the invention and a better appreciation of the advantages of the invention, the following illustrative example is given.

Example A nickel-chromium alloy (hereinafter called Alloy X) containing, by weight, about 18.2% chromium, about 16.6% cobalt, about 2.6% titanium, about 1.32% aluminium, about 0.9% silicon, about 0.15% manganese, about 0.001% lead, about 0.02% zirconium, about 0.005% boron, less than about 0.005 magnesium, about 1.18% iron, about 0.01% copper, less than about 0.0002% bismuth, about 0.078% carbon, less than about 0.005 zinc, about 0.01% tin with the balance essentially nickel was found to have a lattice parameter of 3.567 angstrom units (A.). Another nickel-chromium alloy (hereinafter called Alloy Y) containing, by weight, about 20.9% chromium, about 18.5% cobalt, about 5.56% molybdenum, about 1.2% titanium, about 5.5% aluminum, about 0.16% silicon, less than about 0.1% manganese, about 0.002% lead, less than about 0.01% boron, about 0.005% magnesium, about 0.08% copper, about 0.2% iron, less than about 0.002% bismuth, about 0.01% tin, about 0.01% zinc, about 0.06% carbon with the balance essentially nickel was found to have a lattice parameter of 3.591 (A.). Each alloy was then separately heated until it was in the molten state and passed through an orifice under pressure into a water atmosphere in order to disintegrate the molten metal. This procedure, i.e., water atomization, produced alloy powders upon cooling of the molten metal. The alloy powders were then separately screened on a sieve of 100 mesh (apertures 152 microns) and the material that passed through was then screened on a sieve of 200 mesh (apertures 76 microns). Some of the thus-formed powders of Alloy X and of Alloy Y in a ratio of 3 parts of Alloy X to 1 part of Alloy Y (75% Alloy X and 25% Alloy Y) retained on the 200 mesh sieve but passing through the mesh sieve were then placed in a 6" long mild steel sheath that was closed at one end. The outside diameter of the sheath was 2 /2 and the inside diameter was 1%". The sheath containing the alloy powders was then filled with argon and a mild steel end cap was welded onto the open end to close the sheath and protect the alloy powders from the ambient atmosphere. The sheath was then upset forged to compact the powder into an alloy core. The core was then machined out of the sheath and then machined until it had a cylindrical configuration. The cylindrical compacted alloy was placed in another mild sheath as before, filled with argon, sealed and then extruded at a temperature of about 1160" C. The outer skin was subsequently machined away and then the alloy was machined to leave an alloy rod having a diameter. The rod was then stress-rupture tested under a load of about 4 t.s.i. at a temperature of about 870 C. and found to have a life-to-rupture of 39.5 hours.

When rods of Alloy X and Alloy Y were prepared in the manner hereinbefore set forth and then stress-rupture tested under a load of about 4 t.s.i. at a temperature of about 870 C. (the same test conditions as for the alloy containing 75% Alloy X and 25% Alloy Y), Alloy X was found to have a life-to-rupture of 8.2 hours while Alloy Y was found to have a life-to-rupture of only 2 hours. Thus, the alloy containing the mixture of 75% Alloy X and 25% Alloy Y has considerably better strength at 870 C. than either of the alloys from which it is made.

The present invention is applicable to the production of a vast variety of heterogeneous alloys having a great variety of uses such as turbine blades, structural components and parts for elevated temperature use, etc. The present invention is particularly applicable to the formation of heterogeneous alloys comprising a matrix of an alloy and a dispersed phase of another alloy which latter alloy acts as a slip-inhibiting phase. Moreover, the present invention finds use in the making of wrought heterogeneous 'sintered alloys having those superior strength properties usually associated with cast alloys of like overall composition but without certain disadvantages of cast alloys such as defects due to inter alia entrapped gas and the general dislike of cast alloys in high energy machines where these defects are often associated with low impact strengths. The invention is also broadly applicable to numerous alloys, e.g., nickel-base alloys, cobalt-base alloys and iron-base alloys, provided always that the two or more alloys chosen are compatible with one another as hereinbefore set forth.

Although the present invention has been described in conjunction with preferred embodiments, it is to be un derstood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the present invention and appended claims.

We claim:

1. A process for making strong alloys comprising mixing about 75% of an alloy powder containing, by weight, about 18.2% chromium, about 16.6% cobalt, about 2.6% titanium, about 1.3% aluminium, about 0.9% silicon, about 1.2% iron, about 0.08% carbon with the balance essentially nickel and having a particle size of about 76 microns to about 152 microns with about 25% of an alloy powder containing, by weight, about 20.9% chromium, about 18.5 cobalt, about 5.6% molybdenum, about 1.2% titanium, about 5.5% aluminium, about a L fe- 0.2% iron, about 0.2% silicon, about 0.06% carbon with the balance essentially nickel and having a particle size of about 76 microns to about 152 microns, forming said mixture into a compact and then extruding at a temperature of about 1160 C.

2. A process for making strong alloys comprising mixing about 20% to about 80% of a nickel-chromium alloy powder with about 80% to about 20% of another nickelchromium alloy powder, said alloy powders having the same crystal structure, particle sizes of about 75 microns to about 160 microns, melting points within about 150 C. of each other and lattice parameters of within about 0.1% to about of each other, forming a powder compact from said alloy powders and then sintering and consolidating the compact to produce a heterogeneous alloy.

3. A process for making strong alloys comprising mixing about 20% to about 80% of a nickel-base alloy powder with about 80% to about 20% of another nickelbase alloy powder, said alloy powders having the same crystal structure, particle sizes of about 75 microns to about 160 microns, melting points within about 150 C. of each other and lattice parameters of within about 0.1% to about 5% of each other, forming a powder compact from said alloy powders and then sintering and consolidating the compact to produce a heterogeneous alloy.

4. A heterogeneous alloy containing, by weight, about 20% to about 80% of a homogeneous nickel-chromium alloy formed of a nickel-chromium alloy powder, having a particle size of about 75 to about 160 microns, substantially uniformly dispersed in an metallurgically bonded to about 80% to about of another homogeneous nickel-chromium alloy formed of a nickel-chromium alloy powder having a particle size of about to about 160 microns, said homogeneous alloys having lattice parameters within about 0.1% to about 5% of each other, the same crystal structures and having melting points within about 150 C. of each other.

5. A heterogeneous alloy containing, by weight, about 20% to about of a homogeneous nickel-base alloy formed of a nickel-base alloy powder having a particle size of about 75 to about 160 microns substantially uniformly dispersed in and metallurgically bonded to about 80% to about 20% of another homogeneous nickel-base alloy formed of a nickel-base alloy powder having a particle size of about 75 to about 160 microns, said homogeneous alloy having lattice parameters within about 0.1% to about 5% of each other, the same crystal structures and having melting points within about C. of each other.

References Cited in the file of this patent UNITED STATES PATENTS 2,694,790 Studdens Nov. 16, 1954 2,849,789 Thomson Sept. 2, 1958 2,920,958 Bergh Jan. 12, 1960 OTHER REFERENCES Adler: Precision Metal Molding, May 1954, pp. 54-56 and 8081. 

4. A HETEROGENEOUS ALLOY CONTAINING, BY WEIGHT, ABOUT 20% TO ABOUT 80% OF A HOMOGENEOUS NICKEL-CHROMIUM ALLOY FORMED OF A NICKEL-CHROMIUM ALLOY POWDER, HAVING A PARTICLE SIZE OF ABOUT 75 TO ABOUT 160 MICRONS, SUBSTANTIALLY UNIFORMLY DISPERSED IN AN METHALLURIGICALLY BONDED TO ABOUT 80% TO ABOUT 20% OF ANOTHER HOMOGENEOUS NICKEL-CHROMIUM ALLOY FORMED OF A NICKEL-CHROMIUM ALLOY POWDER HAVING A PARTICLE SIZE OF ABOUT 75 TO ABOUT 160 MICRONS, SAID HOMOGENEOUS ALLOYS HAVING LATTICE PARAMETERS WITHIN ABOUT 0.1% TO ABOUT 5% OF EACH OTHER, THE SAME CRYSTAL STRUCTURES AND HAVING MELTING POINTS WITHIN ABOUT 150%C. OF EACH OTHER. 